Method of manufacturing optical communication system

ABSTRACT

A position of a first module ( 12   a ) with respect to an optical fiber ( 11 ) is determined in accordance with a receiving efficiency at the first module ( 12   a ) with respect to light emitted from the optical fiber ( 11 ). Power of light coupled into the optical fiber ( 11 ) from the first module ( 12   a ) is set in accordance with a value of a far-end reflectivity on a side of the first module ( 12   a ) in the position so as to satisfy a predetermined formula. By giving priority to determining a condition that significantly influences an improvement of an eye opening ratio, it is possible to manufacture an optical communication system at a low cost and with more freedom in manufacturing. Thus provided is a method of manufacturing an optical communication system, the method allowing for manufacturing an optical communication system at a low cost and with more freedom in manufacturing.

This application is a US national stage application of PCT/JP03/04217filed Apr. 2, 2003.

TECHNICAL FIELD

The present invention relates to a method of manufacturing an opticalcommunication system including (i) an optical fiber and (ii) modulesrespectively provided at both ends of the optical fiber, the modulesbeing capable of sending and receiving optical signals simultaneouslyvia the optical fiber.

BACKGROUND ART

Conventionally, a domestic application of single-conductor full-duplextwo-way communication is proposed. FIG. 10 is a conceptual diagramillustrating a domestic application example of the single-conductorfull-duplex two-way communication. In this application example,electronic appliances such as a TV and a PC are connected with eachother by an optical fiber 10 usable for the single-conductor full-duplextwo-way communication, thus establishing a domestic multimedia network.The domestic multimedia network is connected with an outer network via agateway and the like.

Digital broadcasting started in the fiscal year 2000. In a few years, itwill become commonplace that a household is connected via FTTH (Fiber ToThe Home). In order to conform to the FTTH, the optical fiber of thehousehold needs to have a transmission capacity of 100 Mbps at maximum.Approximately the same capacity is required for conforming to thedigital broadcasting. Moreover, communication through network-type gamemachines and digital video editing machines will also be performed viathe optical fiber of the household.

In order to realize such communication principally for high-definitionvideo transmission, domestic communication requires a low-error-rate,high-quality transmission method with a transmission capacity ofseveral-hundred Mbps.

As one such communication method, a domestic network by IEEE1394 isdrawing attention. IEEE1394 supports a long-distance transmission and avery low error rate (lower than 10⁻¹² in BER (Bit Error Rate)).Therefore, IEEE1394 is considered as an excellent method for thedomestic multimedia network. As a medium of the long-distancetransmission, a silica fiber and a POF (Plastic Optical Fiber) areconsidered. Especially, the POF is easy to use because the POF is easyto connect due to a large diameter thereof.

Incidentally, in order to realize the single-conductor full-duplextwo-way communication, various proposals are made as to structures ofmembers used for the single-conductor full-duplex two-way communication.For example, Japanese Publication for Unexamined Patent Publication,Tokukaihei 11-237535 (publication date: Aug. 31, 1999) (publication 1)discloses a structure of an optical sending and receiving device capableof preventing optical crosstalk that can occur while optical signals arebeing sent and received.

FIG. 11 is a perspective view illustrating a schematic arrangement of anoptical sending and receiving device 100 disclosed in publication 1. Theoptical sending and receiving device 100 includes a laser light emittingsource 101, an optical device 102, and a photodiode 103. The laser lightemitting source 101 emits first signal light (laser light) s1. Theoptical device 102 causes the first signal light s1 to be incident intoan end surface 111 a of an optical fiber 111 in such a direction that isdifferent from a direction in which second signal light s2 is emittedfrom the end surface 111 a of the optical fiber 111. The photodiode 103receives the second signal light s2 emitted from the end surface 111 aof the optical fiber 111. The optical sending and receiving device 100has such a structure in which the photodiode 103 is positioned out ofreach of reflected light s3. The reflected light s3 is generated whenthe first signal light s1 incident into the end surface 111 a of theoptical fiber 111 is reflected by the end surface 111 a of the opticalfiber 111. With the foregoing structure, it is possible to preventoptical crosstalk of near-end reflected light noises.

Moreover, attempts are made to identify requirements on a ratio (opticalcrosstalk ratio) between received light and the optical crosstalk, therequirements being for attaining the long-distance transmission and thelow error rate, which are supported by IEEE1394. For example, a modelfor optical crosstalk setting in case full-duplex two-way communicationis performed via a single-conductor POF is disclosed in “OP i. LINK:Optical Transmission Technology to Connect 10 m by IEEE1394”, NikkeiElectronics, Dec. 4, 2000 edition (No. 784) (published on Dec. 4, 2000),pp. 167-176 (publication 2).

According to publication 2, in order to satisfy the requirementBER<10⁻¹², it is necessary that an amplitude of the received light be 19times higher than dispersion of a Gaussian noise. After simulations andexperimental data analyses were conducted, it was found that anamplitude of the optical crosstalk needs to be equal to or lower thanone-fourth of the amplitude of the received light, that is, the opticalcrosstalk ratio needs to be equal to or larger than 6.0 dB.

Incidentally, noises (optical crosstalk noises) in the single-conductorfull-duplex two-way communication includes not only the opticalcrosstalk considered in publication 1 (optical crosstalk caused bynear-end reflection), but also optical crosstalk caused by far-endreflection. Therefore, it is also necessary to suppress the opticalcrosstalk noises caused by the far-end reflection. This is an inherentproblem of the single-conductor full-duplex two-way communication. Iffull-duplex two-way communication is performed via a double-conductorfiber, it is not necessary to consider this problem.

In publication 1, there is description of optical crosstalk of the firstsignal light s1 caused on the end surface of the optical fiber 111.However, optical crosstalk caused by reflected returning light generatedon an emission end of the optical fiber 111 and in a sender-side moduleare not considered.

Therefore, with the art of publication 1, it is not always possible tolower the bit error rate to a desired range.

On the other hand, publication 2 describes the optical crosstalk causedby the reflected returning light generated on the emission end of theoptical fiber 111 and in the sender-side module. However, in publication2, an acceptable amount of optical crosstalk is set uniformly, eventhough there are various parameters.

Therefore, in order to manufacture an optical communication systemaccording to conditions disclosed in publication 2, it is necessary tosatisfy strict requirements in designing an optical system and the likeof each member. As a result, a cost of the optical communication systemincreases.

The present invention was made in light of the foregoing problems. Anobject of the present invention is therefore to provide a method ofmanufacturing an optical communication system, the method being capableof increasing degree of freedom in manufacturing the opticalcommunication system, thereby attaining a lower cost.

DISCLOSURE OF INVENTION

In a method of the present invention for manufacturing an opticalcommunication system including (i) an optical fiber and (ii) first andsecond modules respectively provided at both ends of the optical fiber,the first and second modules being capable of simultaneously sending andreceiving optical signals via the optical fiber, a position of the firstmodule with respect to the optical fiber is determined in accordancewith a receiving efficiency at the first module with respect to lightemitted from the optical fiber, and S1_ is set in accordance with avalue of FR_ in the position so as to satisfy Formula 5:

$\begin{matrix}{\begin{matrix}{{(a){~~~~}{If}\mspace{14mu}{IO}} \geq 0.3} \\{\frac{\left( {1 - {IO}} \right)}{0.7} > \frac{{S\mspace{14mu}\max*\left( {\frac{NR}{R\mspace{14mu}\min} + {{FR\_}*{PT}\mspace{14mu}\max^{2}}} \right)} + \mspace{130mu}{X*\frac{N\;{amp}}{R\mspace{14mu}\min}}}{{S1\_}*{PT}\mspace{14mu}\min}}\end{matrix}\begin{matrix}{{(b){~~~}{If}\mspace{14mu}{IO}} < 0.3} \\{1 > \frac{{S\mspace{14mu}\max*\left( {\frac{NR}{R\mspace{14mu}\min} + {{FR\_}*{PT}\mspace{14mu}\max^{2}}} \right)} + {X*\frac{N\;{amp}}{R\mspace{14mu}\min}}}{{S1\_}*{PT}\mspace{14mu}\min}}\end{matrix}} & (5)\end{matrix}$where FR_ is a far-end reflectivity, which is a reflectivity (a) oflight emitted from the second module and (b) on the first module and onthe first-module-side end of the optical fiber; S1_ is power of lightcoupled into the optical fiber from the first module; Smax is a maximumvalue acceptable in the optical communication system as a value of thepower of light coupled into the optical fiber; PTmin is a minimum valueacceptable in the optical communication system as a transmittance of theoptical fiber with respect to the optical signals; PTmax is a maximumvalue acceptable in the optical communication system as the value of thetransmittance of the optical fiber with respect to the optical signals;NR is a ratio, with respect to Smax, of a stray light component receivedby the second module, the stray light component being generated on thesecond-module-side end of the optical fiber and in the second modulewhen light to be coupled into the optical fiber with power of Smax isemitted from the second module; Rmin is a minimum receiving efficiencyat the second module with respect to light emitted from the opticalfiber; Namp is a light amount corresponding to a noise in an amplifierfor converting, into an electric signal, an optical signal received bythe second module; IO is an eye opening ratio required for the electricsignal obtained by conversion through the amplifier; and X is a ratio,with respect to Namp, of an optical signal received by the second modulewhen a bit error rate is in an upper limit value acceptable in theoptical communication system, where it is assumed that there is noreflected light returning to the second module after being emitted fromthe second module.

With this method, it is possible to determine the position of the firstmodule with priority on the receiving efficiency at the first modulewith respect to the light emitted from the optical fiber. Therefore, itis possible to avoid deterioration of the receiving efficiency at thefirst module. Incidentally, there is a trade-off between the receivingefficiency on a side of the first module and the far-end reflectivityFR_. The far-end reflectivity FR_ tends to increase if the receivingefficiency is improved. If the far-end reflectivity FR_ increases, theeye opening ratio at the second module is adversely affected.

In view of the circumstances, in the foregoing method, the power S1_ ofthe light coupled into the optical fiber from the first module is set inaccordance with the far-end reflectivity FR_ so as to satisfy Formula 5.With this arrangement, it is possible to attain the eye opening ratiorequired for the electric signal obtained by conversion through theamplifier of the second module.

It is for the following reason that priority is given to the receivingefficiency on the side of the first module. The applicant of the presentapplication found that, on an improvement of the eye opening ratio on areceiver side, the receiving efficiency on the receiver side has greaterinfluence than the far-end reflectivity on a sender side. Therefore, ifpriority is given to the receiving efficiency on the side of the firstmodule, and the first module is positioned so as to increase thereceiving efficiency on the side of the first module, a requirement forimproving the eye opening ratio on the side of the first module becomesless strict. That is, with all things considered, an opticalcommunication system can be manufactured under a less strict requirementif priority is given to the receiving efficiency on the sender side(here, the side of the first module) in determining the position of thefirst module.

By thus giving priority to determining a condition that significantlyinfluences the improvement of the eye opening ratio, it is possible tomanufacture an optical communication system at a lower cost and withmore freedom in manufacturing.

In a method of the present invention for manufacturing an opticalcommunication system including (i) an optical fiber and (ii) first andsecond modules respectively provided at both ends of the optical fiber,the first and second modules being capable of simultaneously sending andreceiving optical signals via the optical fiber, a position of the firstmodule with respect to the optical fiber is determined in accordancewith a receiving efficiency at the first module with respect to lightemitted from the optical fiber, and, from plural groups of modules, themodules being different from group to group, a group in which S1min_satisfies Formula 6

$\begin{matrix}{\begin{matrix}{{(a){~~~}{If}\mspace{14mu}{IO}} \geq 0.3} \\{\frac{\left( {1 - {IO}} \right)}{0.7} > \frac{\begin{matrix}{{S\mspace{14mu}\max*\left( {\frac{NR}{R\mspace{14mu}\min} + {{FR\_}*{PT}\mspace{14mu}\max^{2}}} \right)} +} \\{X*\frac{N\;{amp}}{R\mspace{14mu}\min}}\end{matrix}}{{S1min\_}*{PT}\mspace{14mu}\min}}\end{matrix}\begin{matrix}{{(b){~~~}{If}\mspace{14mu}{IO}} < 0.3} \\{1 > \frac{{S\mspace{14mu}\max*\left( {\frac{NR}{R\mspace{14mu}\min} + {{FR\_}*{PT}\mspace{14mu}\max^{2}}} \right)} + {X*\frac{N\;{amp}}{R\mspace{14mu}\min}}}{{S1min\_}*{PT}\mspace{14mu}\min}}\end{matrix}} & (6)\end{matrix}$is selected in accordance with a value of FR_ in the position, andmodules included in the selected group are used as the first module,where FR_ is a far-end reflectivity, which is a reflectivity (a) oflight emitted from the second module and (b) on the first module and onthe first-module-side end of the optical fiber; S1min_ is a minimumvalue among various values of power of light coupled into the opticalfiber from a group of modules of a same kind adoptable as the firstmodule; Smax is a maximum value acceptable in the optical communicationsystem as a value of the power of light coupled into the optical fiber;PTmin is a minimum value acceptable in the optical communication systemas a transmittance of the optical fiber with respect to the opticalsignals; PTmax is a maximum value acceptable in the opticalcommunication system as the value of the transmittance of the opticalfiber with respect to the optical signals; NR is a ratio, with respectto Smax, of a stray light component received by the second module, thestray light component being generated on the second-module-side end ofthe optical fiber and in the second module when light to be coupled intothe optical fiber with power of Smax is emitted from the second module;Rmin is a minimum receiving efficiency at the second module with respectto light emitted from the optical fiber; Namp is a light amountcorresponding to a noise in an amplifier for converting, into anelectric signal, an optical signal received by the second module; IO isan eye opening ratio required for the electric signal obtained byconversion through the amplifier; and X is a ratio, with respect toNamp, of an optical signal received by the second module when a biterror rate is in an upper limit value acceptable in the opticalcommunication system, where it is assumed that there is no reflectedlight returning to the second module after being emitted from the secondmodule.

With this method, instead of individually setting properties of modulesused as the first module, it is possible to consider various propertiesof the modules of the same kind in determining which kind of modules toadopt as the first module. As a result, it is possible to save time andlabor to set the properties of the modules individually.

In a method of the present invention for manufacturing an opticalcommunication system including (i) an optical fiber and (ii) first andsecond modules respectively provided at both ends of the optical fiber,the first and second modules being capable of simultaneously sending andreceiving optical signals via the optical fiber,

-   -   a position of the first module with respect to the optical fiber        is determined in accordance with a receiving efficiency at the        first module with respect to light emitted from the optical        fiber, and PT1_ is set in accordance with a value of FR_ in the        position so as to satisfy Formula 7:

$\begin{matrix}{\begin{matrix}{{(a){~~~}{If}\mspace{14mu}{IO}} \geq 0.3} \\{\frac{\left( {1 - {IO}} \right)}{0.7} > \frac{\begin{matrix}{{S\mspace{14mu}\max*\left( {\frac{NR}{R\mspace{14mu}\min} + {{FR\_}*{PT}\mspace{14mu}\max^{2}}} \right)} +} \\{X*\frac{N\;{amp}}{R\mspace{14mu}\min}}\end{matrix}}{S\mspace{14mu}\min*{PT1\_}}}\end{matrix}\begin{matrix}{{(b){~~~}{If}\mspace{14mu}{IO}} < 0.3} \\{1 > \frac{{S\mspace{14mu}\max*\left( {\frac{NR}{R\mspace{14mu}\min} + {{FR\_}*{PT}\mspace{14mu}\max^{2}}} \right)} + {X*\frac{N\;{amp}}{R\mspace{14mu}\min}}}{S\mspace{14mu}\min*{PT1\_}}}\end{matrix}} & (7)\end{matrix}$where FR_ is a far-end reflectivity, which is a reflectivity (a) oflight emitted from the second module and (b) on the first module and onthe first-module-side end of the optical fiber; PT1_ is a transmitivityof the optical fiber with respect to light emitted from the firstmodule; Smin is a minimum value acceptable in the optical communicationsystem as a value of power of light coupled into the optical fiber; Smaxis a maximum value acceptable in the optical communication system as avalue of the power of light coupled into the optical fiber; PTmax is amaximum value acceptable in the optical communication system as a valueof a transmittance of the optical fiber with respect to the opticalsignals; NR is a ratio, with respect to Smax, of a stray light componentreceived by the second module, the stray light component being generatedon the second-module-side end of the optical fiber and in the secondmodule when light to be coupled into the optical fiber with power ofSmax is emitted from the second module; Rmin is a minimum receivingefficiency at the second module with respect to light emitted from theoptical fiber; Namp is a light amount corresponding to a noise in anamplifier for converting, into an electric signal, an optical signalreceived by the second module; IO is an eye opening ratio required forthe electric signal obtained by conversion through the amplifier; and Xis a ratio, with respect to Namp, of an optical signal received by thesecond module when a bit error rate is in an upper limit valueacceptable in the optical communication system, where it is assumed thatthere is no reflected light returning to the second module after beingemitted from the second module.

With this method, it is possible to attain the eye opening ratiorequired for the electric signal obtained by conversion through theamplifier of the second module, by adjusting the transmittance PT1_ ofthe optical fiber with respect to the light emitted from the firstmodule, instead of adjusting the power S1_ of the light coupled into theoptical fiber from the first module.

In a method of the present invention for manufacturing an opticalcommunication system including (i) an optical fiber and (ii) first andsecond modules respectively provided at both ends of the optical fiber,the first and second modules being capable of simultaneously sending andreceiving optical signals via the optical fiber, a position of the firstmodule with respect to the optical fiber is determined in accordancewith a receiving efficiency at the first module with respect to lightemitted from the optical fiber, and from plural groups of modules, themodules being different from group to group, a group in which PT1min_satisfies formula 8

$\begin{matrix}{\begin{matrix}{{(a){~~~}{If}\mspace{14mu}{IO}} \geq 0.3} \\{\frac{\left( {1 - {IO}} \right)}{0.7} > \frac{\begin{matrix}{{S\mspace{14mu}\max*\left( {\frac{NR}{R\mspace{14mu}\min} + {{FR\_}*{PT}\mspace{14mu}\max^{2}}} \right)} +} \\{X*\frac{N\;{amp}}{R\mspace{14mu}\min}}\end{matrix}}{S\mspace{14mu}\min*{PT1min\_}}}\end{matrix}\begin{matrix}{{(b){~~~}{If}\mspace{14mu}{IO}} < 0.3} \\{1 > \frac{{S\mspace{14mu}\max*\left( {\frac{NR}{R\mspace{14mu}\min} + {{FR\_}*{PT}\mspace{14mu}\max^{2}}} \right)} + {X*\frac{N\;{amp}}{R\mspace{14mu}\min}}}{S\mspace{14mu}\min*{PT1min\_}}}\end{matrix}} & (8)\end{matrix}$is selected in accordance with a value of FR_ in the position, andmodules included in the selected group are used as the first module,where FR_ is a far-end reflectivity, which is a reflectivity (a) oflight emitted from the second module and (b) on the first module and onthe first-module-side end of the optical fiber; PT1min_ is a minimumvalue among various values of a transmittance of the optical fiber withrespect to light emitted from a group of modules of a same kindadoptable as the first module; Smin is a minimum value acceptable in theoptical communication system as a value of power of light coupled intothe optical fiber; Smax is a maximum value acceptable in the opticalcommunication system as a value of the power of light coupled into theoptical fiber; PTmax is a maximum value acceptable in the opticalcommunication system as a value of a transmittance of the optical fiberwith respect to the optical signals; NR is a ratio, with respect toSmax, of a stray light component received by the second module, thestray light component being generated on the second-module-side end ofthe optical fiber and in the second module when light to be coupled intothe optical fiber with power of Smax is emitted from the second module;Rmin is a minimum receiving efficiency at the second module with respectto light emitted from the optical fiber; Namp is a light amountcorresponding to a noise in an amplifier for converting, into anelectric signal, an optical signal received by the second module; IO isan eye opening ratio required for the electric signal obtained byconversion through the amplifier; and X is a ratio, with respect toNamp, of an optical signal received by the second module when a biterror rate is in an upper limit value acceptable in the opticalcommunication system, where it is assumed that there is no reflectedlight returning to the second module after being emitted from the secondmodule.

With this method, instead of individually setting properties of modulesused as the first module, it is possible to consider various propertiesof the modules of the same kind in determining which kind of modules toadopt as the first module. As a result, it is possible to save time andlabor to set the properties of the modules individually.

In the foregoing methods for manufacturing an optical communicationsystem, it is more suitable that the optical fiber is a plastic opticalfiber, which is generally larger in diameter.

For a fuller understanding of the nature and advantages of theinvention, reference should be made to the ensuing detailed descriptiontaken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a conceptual diagram illustrating a relationship betweensignal light and reflected returning light (noise light) insingle-conductor full-duplex two-way communication.

FIG. 2 is a diagram illustrating an eye pattern for explaining an eyeopening ratio required.

FIG. 3 is a graph illustrating a relationship between a signal-noiseratio SN and a fiber length of an optical fiber.

FIG. 4 is a graph illustrating a relationship between the signal-noiseratio SN and the fiber length of the optical fiber.

FIG. 5 is a graph illustrating a relationship between a sender-sidefiber-coupled optical power and an acceptable far-end reflectivity.

FIG. 6 is a graph illustrating a relationship between a sent lighttransmittance and the acceptable far-end reflectivity.

FIG. 7 is a graph illustrating a relationship between a receivingefficiency and a far-end reflectivity at a receiver.

FIGS. 8( a) and 8(b) are diagrams explaining with illustration thatthere is a trade-off between (i) the receiving efficiency at thereceiver with respect to light emitted from the optical fiber and (ii)the far-end reflectivity.

FIGS. 9( a) and 9(b) are graphs showing influences of the receivingefficiency and the far-end reflectivity on the eye opening ratio IO.

FIG. 10 is a conceptual diagram illustrating a domestic applicationexample of the single-conductor full-duplex two-way communication.

FIG. 11 is a perspective view illustrating a schematic arrangement of aconventional optical sending and receiving device.

BEST MODE FOR CARRYING OUT THE INVENTION

With reference to FIGS. 1 to 9( b), the following describes oneembodiment of the present invention.

FIG. 1 is a conceptual diagram illustrating a relationship betweensignal light and reflected returning light in the single-conductorfull-duplex two-way communication.

A single-conductor full-duplex two-way optical communication system 10(optical communication system) of the present embodiment can be used forperforming (i) domestic communication, (ii) communication betweenelectronic appliances, (iii) LAN (Local Area Network), and the like, viaa multimode optical fiber (e.g. a POF) as a transmission medium.

With reference to FIG. 1, optical crosstalk caused by reflectedreturning light generated on an end surface of an optical fiber and in asender-side module is described in detail.

In order to perform single-conductor full-duplex two-way communication,the single-conductor full-duplex two-way optical communication system 10includes an optical fiber 11 (e.g. a POF), and first and second modules12 a and 12 b (optical communication modules) that are respectivelyprovided at both end surfaces 11 a and 11 b of the optical fiber 11. Thefirst and second modules 12 a and 12 b are capable of sending andreceiving optical signals simultaneously via the optical fiber 11. Thefirst module 12 a includes a sending section 13 a and a receivingsection 14 a. The sending section 13 a sends (emits) signal light s1 tothe second module 12 b, and the receiving section 14 a receives signallight s2 emitted from the second module 12 b. Likewise, the secondmodule 12 b includes a sending section 13 b and a receiving section 14b; the sending section 13 b sends the signal light s2 to the firstmodule 12 a, and the receiving section 14 b receives the signal light s1emitted from the first module 12 a.

Thus, in the single-conductor full-duplex two-way optical communicationsystem 10, both the first module 12 a and the second module 12 b arecapable of sending and receiving signal light. However, for the purposeof explanation, the following discussion focuses on how the signal lights1 sent from the first module 12 a and received by the second module 12b is influenced by optical crosstalk.

The signal light s1 is emitted from the sending section 13 a of thefirst module 12 a, transmitted through the optical fiber 11, andreceived by the receiving section 14 b of the second module 12 b. At thesame time, in order to perform full-duplex communication, the secondmodule 12 b emits the signal light s2 to the first module 12 a whilereceiving the signal light s1. The signal light s2 causes the opticalcrosstalk, which is a noise to the signal light s1 to be received by thesecond module 12 b. The optical crosstalk is caused as reflectedreturning light generated at each interface between the first module 12a and the second module 12 b.

Specifically, optical crosstalk at the receiving section 14 b of thesecond module 12 b includes three kinds of reflected returning light n1,n2, and n3. The reflected returning light n1 is generated from thesignal light s2 on the end surface 11 b, which is an incident side, ofthe optical fiber 11. The reflected returning light n2 is generated fromthe signal light s2 on the end surface 11 a, which is an emission side,of the optical fiber 11. The reflected returning light n3 is generatedfrom the signal light s2 in the first module 12 a. The reflectedreturning light n1 is called “near-end reflection”, and the reflectedreturning light n2 and n3 are called “far-end reflection”. Opticalcrosstalk to the signal light s1 is a sum of the reflected returninglight n1 to n3.

In order to satisfy the requirement BER<10⁻¹² in the single-conductorfull-duplex two-way communication, it is necessary to suppress the sumof the reflected returning light n1 to n3 to be smaller than apredetermined ratio with respect to a signal component that derives fromthe signal light s1, the reflected returning light n1 to n3 beinggenerated from the signal light s2, which is for the sender-side module.

In publication 2, which is described in BACKGROUND ART section, amaximum value of the crosstalk caused by the far-end reflection is setuniformly. However, in order to design the single-conductor full-duplextwo-way optical communication system 10 so as to attain the uniformlyset value of the far-end reflection, design margin of the first and thesecond modules 12 a and 12 b becomes small. As a result, degree offreedom in optical design is decreased. When the degree of freedom inoptical design is decreased, constraints in designing commercialproducts are increased. This increases a cost of the single-conductorfull-duplex two-way optical communication system 10. Moreover, if such aspecification as to uniformly set the maximum value of the opticalcrosstalk caused by the far-end reflection is adopted, the foregoingproblems will prevent other business enterprises from adjusting theirproducts to the specification. As a result, promotion of thespecification will be difficult.

In view of the circumstances, the following discusses requirements forattaining BER<10⁻¹² in the single-conductor full-duplex two-way opticalcommunication system 10 without lowering the degree of freedom inoptical design.

First, the reflected returning light n1 to n3 are neglected. If there isno optical crosstalk caused by the reflected returning light n1 to n3,it is necessary that a signal light amount S and an amplifier noise Nampsatisfy Formula 9:

$\begin{matrix}{\frac{S}{N\;{amp}} > 14.1} & (9)\end{matrix}$Here, the signal light amount S is an amount of signal light coupledinto the photodiode (PD) provided to the receiving section 14 b of thesecond module 12 b, and the amplifier noise Namp is an rms value of anoise current generated by a first-stage preamplifier, the rms valuebeing expressed in light amount.

In Formula 9, it is supposed that the single-conductor full-duplextwo-way optical communication system 10 satisfies BER<10⁻¹². Here, if(i) an optical signal and an electric signal are considered to beequivalent (that is, if an amplitude of received light and an amplitudeof an electric signal obtained by converting the received light throughthe photodiode are considered to be equivalent), and (ii) whether theoptical signal represents “0” or “1” in a binary signal is judged from ahalf level of the amplitude of the electric signal corresponding to theoptical signal, BER of the optical signal is generally given by thefollowing Formula 10:

$\begin{matrix}{{BER} = {0.5*{\left( {1 - {\frac{2}{\sqrt{\pi}}{\int_{0}^{\frac{SN}{2*\sqrt{2}}}{{\mathbb{e}}^{- t^{2}}{\mathbb{d}t}}}}} \right).}}} & (10)\end{matrix}$Note that “*” in the present description is a multiplication sign.

A signal-noise ratio SN is a ratio S/Namp between the signal lightamount S and the amplifier noise Namp. From Formula 10, BER<10⁻¹² issatisfied when the signal-noise ratio SN is 14.1 (11.5 dB). When thesignal-noise ratio SN is 14.1, the ratio expressed in electric power is14.1²≈200 (approximately 23 dB).

The present invention may be applied to cases other than BER<10⁻¹². In ageneralized expression, BER is given by Formula 11. When BER is 10⁻⁸,10⁻¹⁰, 10⁻¹¹, 10⁻¹², or 10⁻¹³, X is 11.1, 12.0, 12.8, 13.5, 14.1, or14.7, respectively.

$\begin{matrix}{\frac{S}{N\;{amp}} > X} & (11)\end{matrix}$

In reality, the signal light amount S cannot be constant during a fullunit interval (UI). If an NRZ encoding, which is often used as anencoding rule for optical fiber communication, is used, it is requiredthat a −3 dB bandwidth of the system be at least twice a bit rate. Forexample, if a transmission rate of the system is 250 Mbps, a bandwidthrequired is 125 MHz. If the transmission rate of the system is 125 MHz,a rising and falling speed of an eye pattern of the system is 2.8 nsecat 0.35/125 MHz. Moreover, a minimum phase margin required isapproximately 0.4 UI, the phase margin being transmitted from an opticaltransceiver to a physical layer LSI provided downstream. Because the UIof 250 Mbps is 4 nsec, a maximum signal amplitude cannot be attainedthroughout 0.4 UI. As shown in FIG. 2, in general, a steepest edgeportion of an eye opening is influenced by a taper of rise and fall.Basically, a substantive amplitude A of an eye opening edge portion isexpressed as (1-IO)/2/0.35*S, where IO is an eye opening ratio required.When the eye opening ratio required is 0.4 UI as in the foregoingexample, the amplitude A of the eye opening edge portion is 6/7 of thesignal light amount S.

Therefore, when the eye opening ratio IO required is not less than 0.3,Formula 11 is expressed as Formula 12:

$\begin{matrix}{\frac{S*{\left( {1 - {IO}} \right)/0.7}}{N\;{amp}} > X} & (12)\end{matrix}$

Next, the reflected returning light n1 to n3 are taken intoconsideration. If there is optical crosstalk caused by the reflectedreturning light n1 to n3, noises can be conceived as a loss of an amountof the optical signal, the loss being equal to amounts of the reflectedreturning light n1 to n3. Therefore, in the end, the following Formula13 needs to be satisfied:

$\begin{matrix}\begin{matrix}\begin{matrix}{{(a){~~~}{If}\mspace{14mu}{IO}} \geq 0.3} \\{\frac{{S*{\left( {1 - {IO}} \right)/0.7}} - \left( {{N1} + {N2} + {N3}} \right)}{N\;{amp}} > X}\end{matrix} \\\begin{matrix}{{(b){~~~}{If}\mspace{14mu}{IO}} < 0.3} \\{\frac{S - \left( {{N1} + {N2} + {N3}} \right)}{N\;{amp}} > X}\end{matrix}\end{matrix} & (13)\end{matrix}$where N1, N2, and N3 are the amounts of the reflected returning light n1to n3, respectively.

Usually, the signal-noise ratio becomes a worst ratio when the signallight amount S is the smallest, and the amplifier noise Namp generatedby the preamplifier becomes dominant among electric noises. With respectto this noise current, considering rising and falling time of the systemas described above, it is necessary to keep, to a certain value or more,a value obtained by subtracting the light amounts N1, N2, and N3 of thereflected returning light n1 to n3 from the signal light amount S.

The following explains meaning of signs used in this description.

Amplifier noise Namp: A light amount corresponding to an rms value ofthe noise current, the noise current being generated by the first-stagepreamplifier of the receiving section 14 b of the second module 12 b.

Signal-noise ratio limit value X: A limit value of the signal-noiseratio SN that attains predetermined BER if there is no reflectedreturning light.

Signal light amount S: An amount of signal light emitted from theoptical fiber 11 and coupled into the photodiode of the receivingsection 14 b of the second module 12 b. The signal light amount S can bemeasured by receiving, at an optical power meter or the like, all lightbeams of emitted light.

Acceptable minimum fiber-coupled optical power Smin: A minimum valueacceptable as power of light coupled into the optical fiber 11 in thesingle-conductor full-duplex two-way optical communication system 10.

Acceptable maximum fiber-coupled optical power Smax: A maximum valueacceptable as the power of light coupled into the optical fiber 11 inthe single-conductor full-duplex two-way optical communication system10.

Sender-side fiber-coupled optical power S1: An amount of light coupledinto the optical fiber 11, the light coupled into the optical fiber 11being a part of the signal light s1 transmitted from the sending section13 a of the first module 12 a (the sender side). The sender-sidefiber-coupled optical power S1 can be measured by coupling the signallight s1 into one end of an optical fiber (which is so short, e.g. 1 m,that decay is ignorable), and measuring an amount of light emitted fromthe other end of the optical fiber.

Sender-side minimum fiber-coupled optical power S1min: A minimum valueamong various values of the sender-side fiber-coupled optical power S1,the various values deriving from various properties of identical lightsources and identical optical systems used in a group of the firstmodule 12 a.

Acceptable minimum transmittance PTmin: A minimum transmittanceacceptable in the single-conductor full-duplex two-way opticalcommunication system 10.

Acceptable maximum transmittance PTmax: A maximum transmittanceacceptable in the single-conductor full-duplex two-way opticalcommunication system 10.

Sent light transmittance PT1: An efficiency indicated by an amount oflight transmitted through the optical fiber 11, the light transmittedthrough the optical fiber 11 being a part of light sent from the sendingsection 13 a of the first module 12 a (the sender side) and coupled intothe optical fiber 11. The sent light transmittance PT1 can be obtainedby coupling light, with identical fiber-coupled optical power, intooptical fibers having different lengths, and measuring amounts of lightemitted from the respective optical fibers.

Sent light minimum transmittance PT1min: A minimum value among variousvalues of the sent light transmittance PT1 with respect to light (thesignal light s1) sent from a group of the first module 12 a usingidentical light sources and identical optical systems, the variousvalues deriving from various properties of the identical light sourcesand identical optical systems used in the group of the first module 12a.

Near-end reflectivity NR: A value indicating an amount of innerscattered light (a stray light component), such as leaked and scatteredlight, generated on the end surface 11 b and in the second module 12 b,and received by the receiving section 14 b of the second module 12 b,the inner scattered light being a part of the signal light s2 sent fromthe second module 12 b. The near-end reflectivity NR is a ratio of theamount of the inner scattered light received, with respect to theacceptable maximum fiber-coupled optical power Smax.

Far-end reflectivity FR: A sum of a reflectivity PFR and a reflectivityMFR. The reflectivity PFR is a reflectivity of the signal light s2 sentfrom the second module 12 b and reflected on the end surface 11 a, i.e.the end surface on the side of the first module 12 a. The reflectivityMFR is a reflectivity of the signal light s2 sent from the second module12 b and reflected by the first module 12 a, which is the sender-sidemodule.

Minimum receiving efficiency Rmin: A minimum value of a ratio of thelight coupled into the photodiode of the receiving section 14 b of thesecond module 12 b, with respect to the light emitted from the opticalfiber 11.

Full unit interval UI (Unit Interval): A unit interval of the system.

Eye opening ratio IO required: An eye opening ratio required where thefull unit interval UI is 1.

Eye opening edge portion amplitude A: A signal amplitude at a phasemargin edge portion of the eye pattern.

Note that, when something is “acceptable in the single-conductorfull-duplex two-way optical communication system 10”, it is accepted bya specification or the like of the single-conductor full-duplex two-wayoptical communication system 10.

First Embodiment

In order to connect devices that are in the same room or in adjacentrooms, 10 m is a sufficient wire length. Here, a 10 m-length SI-type(Step Index type) plastic optical fiber MH4001 (a product of MitsubishiRayon Co., Ltd) is used as the optical fiber 11 of the single-conductorfull-duplex two-way optical communication system 10. Discussed below isa case in which the first module 12 a has a 125 Mbps transmissioncapacity, which is relatively low, and the second module 12 b has a 250Mbps transmission capacity, which is relatively high. A phase marginrequired by the physical-layer LSI provided downstream is 0.4 UI, whichis a common value.

Principal constituents are as follows. In the first module 12 a (whichhas the 125 Mbps transmission capacity), (i) the photodiode of thereceiving section 14 a is a high-speed PIN-PD (a product of SharpKabushiki Kaisha) whose light receiving section has a ø440 μm diameter,(ii) the preamplifier is an fc125 MHz product (a product of SharpKabushiki Kaisha) whose amplification noise is 55 nA, (iii) a sender ofthe sending section 13 a is a red LED (a product of Sharp KabushikiKaisha), and (iv) an LED driver is an fc125 MHz product (a product ofSharp Kabushiki Kaisha). In the second module 12 b (which has the 250Mbps transmission capacity), (i) the photodiode of the receiving section14 b is a high-speed PIN-PD (a product of Sharp Kabushiki Kaisha) whoselight receiving section has a ø350 μm diameter, (ii) the preamplifier isan fc125 MHz product (a product of Sharp Kabushiki Kaisha) whoseamplification noise is 98 nA, (iii) a sender of the sending section 13 bis a red LED (RIN-120 dB, a product of Sharp Kabushiki Kaisha), and (iv)an LED driver is Max3766 (a product of Maxim).

Values of principal parameters that determine the signal-noise ratio SNare as follows. A minimum transmittance of the optical fiber 11 per unitlength is −0.51 dB/m (in case the sender is an LED). A maximumtransmittance of the optical fiber 11 per unit length is −0.15 dB/m (incase the sender is an LED). The acceptable maximum fiber-coupled opticalpower Smax acceptable in the single-conductor full-duplex two-wayoptical communication system 10 is −2.7 dB. The minimum fiber-coupledoptical power Smin acceptable in the single-conductor full-duplextwo-way optical communication system 10 is −10.9 dB. A worst value of areceiving coupling efficiency between (i) the optical fiber 11 and (ii)the receiving sections 14 a and 14 b is −8 dB. The near-end reflectivityof the reflected returning light n1 is 0.1% (with respect to the Smax),and the far-end reflectivity of the reflected returning light n1 and n2is 1.4%. A minimum quenching ratio of the single-conductor full-duplextwo-way optical communication system 10 is 10.

The signal-noise ratio SN becomes a worst ratio when an optical signalis sent from (i) the 125 Mbps first module 12 a using the LED with whichthe minimum transmittance is −51 dB/m to (ii) the 250 Mbps second module12 b using the LED with which the maximum transmittance is −0.15 dB/m.FIG. 3 shows the signal-noise ratio with respect to each fiber length ofthe optical fiber 11 under the foregoing worst combination. Thesignal-noise ratio SN becomes the worst ratio when the fiber length ofthe optical fiber 11 is 10 m. Nonetheless the signal-noise ratio SNsatisfies a desired value of not lower than 23 dB.

The signal-noise ratio SN can be improved by increasing optical power ofthe LD or the LED, that is, by relatively raising the fiber-coupledoptical power. However, because the single-conductor full-duplex two-wayoptical communication system 10 here is for household use, it isnecessary to consider safety for the eyes, power consumption, life ofthe light-emitting element, and the like. Therefore, an average value ofthe fiber-coupled optical power is at most approximately −2.7 dBm.Incidentally, the minimum value of the fiber-coupled optical power isapproximately −10.9 dBm, considering that APC and the like cannot beperformed in case of the LED, and that the fiber-coupled optical powervaries.

A worst value of the receiving coupling efficiency is approximately −8dB, considering that the diameter of the optical fiber 11 is larger thanthe diameter (1 mm) of the receiving section of the photodiode, and anNA of light emitted from the optical fiber 11 is large in case the sentlight is LED light, and considering, for example, a loss incurred whenthe optical signals sent and received are divided by a wave division ora polarization division.

The far-end reflectivity FR can be decreased to some extent by anend-surface slanting process or a spherical process in case the opticalfiber 11 is the SI-type plastic optical fiber, for example. In thiscase, the far-end reflectivity of the optical fiber 11 can be suppressedto approximately 0.7%. Moreover, through an experiment, it was confirmedthat a far-end reflectivity on the sender-side can also be suppressed toapproximately 0.7% by slanting the element, for example. Therefore, thefar-end reflectivity FR of the reflected returning light n2 and n3 isset to 1.4%.

An appropriate value of the near-end reflectivity NR is 1/1000 of theacceptable maximum fiber-coupled optical power Smax, consideringscattering and the like incurred on the incident end of the opticalfiber 11.

In this case, the signal-noise ratio expressed in electric power is notlower than 23 dB, even after the optical signal is transmitted 10 m.Thus, it is possible to attain an excellent transmission quality thatsatisfies BER<10⁻¹².

On the other hand, if the far-end reflectivity FR is 2.4%, thesignal-noise ratio SN becomes 20.5 dB after the optical signal istransmitted 10 m, as shown in FIG. 4. As a result, the desired value of23 dB cannot be satisfied.

Discussed below is a case in which a signal is transmitted from thefirst module 12 a to the second module 12 b in the single-conductorfull-duplex two-way optical communication system 10.

Formula 13(a) can be transformed into Formula 14. Here, it is necessaryto set the signal light amount S so that the signal-noise ratio becomesthe worst ratio. Therefore, the signal light amount S, which is theamount of light coupled into the photodiode provided to the receivingsection 14 b of the second module 12 b, is given by Formula 15.

$\begin{matrix}{{\left( {1 - {IO}} \right)/0.7} > {\frac{\left( {{N1} + {N2} + {N3}} \right)}{S} + {X*\frac{N\;{amp}}{S}}}} & (14)\end{matrix}$S=Smin*PTmin*Rmin   (15)

The acceptable minimum transmittance Ptmin is a minimum transmittance ofthe optical fiber 11 acceptable in the single-conductor full-duplextwo-way optical communication system 10. The acceptable minimumtransmittance Ptmin is the minimum transmittance per unit lengthmultiplied by the length of the optical fiber 11.

The minimum receiving efficiency Rmin is the ratio of the light coupledinto the photodiode, the light coupled into the photodiode being a partof the light emitted from the optical fiber 11. The minimum receivingefficiency Rmin is given by (the amount of the light coupled into thephotodiode)/(the amount of the light emitted from the optical fiber).

From FIG. 1, the sum (N1+N2+N3) of the reflected returning light n1 ton3, which are causes of the optical crosstalk, is given by Formula 16when an amount of the signal light emitted from the receiver side is thestrongest.N1+N2+N3=Smax*((MFR+PFR)*PTmax ² *Rmin)+NR)   (16)

The acceptable maximum transmittance PTmax is a maximum transmittance ofthe optical fiber 11 acceptable in the single-conductor full-duplextwo-way optical communication system 10. The acceptable maximumtransmittance PTmax is the maximum transmittance per unit lengthmultiplied by the length of the optical fiber 11.

By plugging Formula 16 into Formula 14 and then simplifying Formula 14,Formula 17(a) is obtained. Likewise, Formula 17(b) is obtained bytransforming Formula 13(b). In order to cause the BER to be smaller thanthe predetermined value, Formula 17 needs to be satisfied.

$\begin{matrix}\begin{matrix}{{(a)\mspace{14mu}{If}\mspace{14mu}{IO}} \geq 0.3} \\{\mspace{34mu}{\frac{\left( {1 - {IO}} \right)}{0.7} > \frac{{S\mspace{11mu}\max*\left( {\frac{NR}{R\mspace{11mu}\min} + {{FR}*{PT}\mspace{11mu}\max^{2}}} \right)} + {X*\frac{Namp}{\;{R\mspace{11mu}\min}}}}{S\mspace{11mu}\min*{PT}\mspace{11mu}\min}}} \\{{(b)\mspace{14mu}{If}\mspace{14mu}{IO}} < 0.3} \\{\mspace{34mu}{1 > \frac{{S\mspace{11mu}\max*\left( {\frac{NR}{R\mspace{11mu}\min} + {{FR}*{PT}\mspace{11mu}\max^{2}}} \right)} + {X*\frac{Namp}{\;{R\mspace{11mu}\min}}}}{S\mspace{11mu}\min*{PT}\mspace{11mu}\min}}}\end{matrix} & (17)\end{matrix}$

In Formulas 17(a) and 17(b), (i) the denominators of the respectiveright sides and (ii) the far-end reflectivity FR are determinedprincipally by the first module 12 a, and the numerators except thefar-end reflectivity FR are determined principally by the second module12 b.

In general, the acceptable maximum fiber-coupled optical power Smax andthe acceptable minimum fiber-coupled optical power Smin are determinedby the specification of the single-conductor full-duplex two-way opticalcommunication system 10. The acceptable maximum transmittance PTmax andthe acceptable minimum transmittance PTmin depend on a wavelength of alight source used, and on an excitation NA of the light source used.Usually, the wavelength and the excitation NA are also determined by thespecification. Therefore, in the single-conductor full-duplex two-wayoptical communication system 10 that satisfies the specification, thefiber-coupled optical light power is within a range between theacceptable minimum fiber-coupled optical power Smin and the acceptablemaximum fiber-coupled optical power Smax, and the transmittance of theoptical fiber 11 is within a range between the acceptable minimumtransmittance PTmin and the acceptable maximum transmittance PTmax.

In order to cause the signal-noise ratio to be equal to or higher thanthe predetermined value, it is necessary to satisfy the followingcondition: (1) a ratio NR/Rmin and a ratio Namp/Rmin are smaller than acertain value on the receiver side, that is, on the side of the secondmodule 12 b, the ratio NR/Rmin being a ratio between the near-endreflectivity NR and the minimum receiving efficiency Rmin, and the ratioNamp/Rmin being a ratio between the amplifier noise Namp and the minimumreceiving efficiency Rmin, or (2) the far-end reflectivity FR is lowerthan a certain value on the sender side, that is, on the side of thefirst module 12 a.

Here, the single-conductor full-duplex two-way optical communicationsystem 10 as a whole is considered. If the far-end reflectivity on thesender side is already determined, it is necessary to set NR/Rmin so asto satisfy Formula 17 under the conditions that the fiber-coupledoptical power is within the range between the acceptable minimumfiber-coupled optical power Smin and the acceptable maximumfiber-coupled optical power Smax, and that the transmittance of theoptical fiber 11 is within the range between the acceptable minimumtransmittance PTmin and the acceptable maximum transmittance PTmax.While NR/Rmin can be adjusted freely as long as the foregoing conditionsare satisfied, the far-end reflectivity FR cannot be adjusted on thereceiver side. Therefore, it is necessary to set the far-endreflectivity FR on the sender side to be lower than the certain value.

In the single-conductor full-duplex two-way optical communication system10 of the present embodiment, (i) the near-end reflectivity NR isapproximately 0.1% due to scattering and the like caused when the signallight s2 is incident into the optical fiber 11, (ii) in a worst case, alower limit of the minimum receiving efficiency Rmin is approximately20%, due to (a) a difference between a fiber diameter of the photodiodeand that of the optical fiber 11, (b) an efficiency of dividing theoptical signals sent and received, (c) a radial pattern of the lightemitted from the optical fiber 11, and the like, and (iii) NR/Rmin is0.005.

As described above, it is necessary to set the far-end reflectivity FRon the sender side to be lower than the certain value. In case of thesingle-conductor full-duplex two-way optical communication system 10 inwhich the optical fiber is short and the transmittance of the opticalfiber 11 is high, it is necessary to set the far-end reflectivity FR onthe sender side to be a very low value. As a result, freedom of moduledesign is narrowed.

In view of the circumstances, the following conditions are set in thepresent embodiment. If the far-end reflectivity FR cannot be lower thanthe predetermined value in formula 17, the far-end reflectivity FR isconsidered as a variable. Because the far-end reflection is caused onthe sender side (here, on the side of the first module 12 a), it ispossible to consider that the sent signal per se includes a noise.

By increasing the sender-side fiber-coupled optical power S1 by anamount of the noise, it is possible to cancel the noise increased by thefar-end reflection. “Smin” in Formula 17 can be set freely as long asthe fiber-coupled optical power S1 is within the range between Smin toSmax, the range being acceptable in the single-conductor full-duplextwo-way optical communication system 10. Therefore, it is possible toconsider “Smin” in Formula 17 as an adjustable variable (sender-sidefiber-coupled optical power S1_).

As a result, FIG. 17 can be expressed as FIG. 18. In FIG. 18, “_”attached after a sign indicates that the sign is a variable.

$\begin{matrix}\begin{matrix}{{(a)\mspace{14mu}{If}\mspace{14mu}{IO}} \geq 0.3} \\{\mspace{34mu}{\frac{\left( {1 - {IO}} \right)}{0.7} > \frac{{S\mspace{11mu}\max*\left( {\frac{NR}{R\mspace{11mu}\min} + {{FR\_}*{PT}\mspace{11mu}\max^{2}}} \right)} + {X*\frac{Namp}{\;{R\mspace{11mu}\min}}}}{{S1\_}*{PT}\mspace{11mu}\min}}} \\{{(b)\mspace{14mu}{If}\mspace{14mu}{IO}} < 0.3} \\{\mspace{34mu}{1 > \frac{{S\mspace{11mu}\max*\left( {\frac{NR}{R\mspace{11mu}\min} + {{FR\_}*{PT}\mspace{11mu}\max^{2}}} \right)} + {X*\frac{Namp}{\;{R\mspace{11mu}\min}}}}{S\; 1\_\;*{PT}\mspace{11mu}\min}}}\end{matrix} & (18)\end{matrix}$

Therefore, by adjusting the sender-side fiber-coupled optical power S1in accordance with a value of the far-end reflectivity FR, a relativelylarge far-end reflection becomes acceptable. In the foregoing specificexample, by simply setting the sender-side fiber-coupled optical powerS1 to −10.5 dBm, a maximum acceptable value of the far-end reflectivityFR is increased to 2%.

Instead of the sender-side fiber-coupled optical power S1, theacceptable minimum transmittance PTmin may be improved by changing theexcitation NA of, or managing a wavelength of, the light (signal lights1) sent from the sending section 13 a of the first module 12 a, whichis the sender side. When “PTmin” in formula 17 is considered as anadjustable variable (sent light transmittance PT1_), Formula 17 can beexpressed as Formula 19.

$\begin{matrix}\begin{matrix}{{(a)\mspace{14mu}{If}\mspace{14mu}{IO}} \geq 0.3} \\{\mspace{34mu}{\frac{\left( {1 - {IO}} \right)}{0.7} > \frac{{S\mspace{11mu}\max*\left( {\frac{NR}{R\mspace{11mu}\min} + {{FR\_}*{PT}\mspace{11mu}\max^{2}}} \right)} + {X*\frac{Namp}{\;{R\mspace{11mu}\min}}}}{{S\mspace{11mu}\min*{PT1\_}}\;}}} \\{{(b)\mspace{14mu}{If}\mspace{14mu}{IO}} < 0.3} \\{\mspace{34mu}{1 > \frac{{S\mspace{11mu}\max*\left( {\frac{NR}{R\mspace{11mu}\min} + {{FR\_}*{PT}\mspace{11mu}\max^{2}}} \right)} + {X*\frac{Namp}{\;{R\mspace{11mu}\min}}}}{S\mspace{11mu}\min*{PT1\_}}}}\end{matrix} & (19)\end{matrix}$

FIG. 5 is a graph showing the far-end reflectivity FR with respect todifferent values of the sender-side fiber-coupled optical power S1_.FIG. 6 is a graph showing the far-end reflectivity FR with respect todifferent values of the sent light transmittance PT1_. In FIGS. 5 and 6,the far-end reflectivity FR is determined so as to satisfy the conditionthat the signal-noise ratio is 14.1 (200 in a optical power and inelectric power). It is found that the acceptable far-end reflectivityincreases as the sender-side fiber-coupled optical power S1_ or the sentlight transmittance PT1_ increases.

Therefore, in a method of the present embodiment for manufacturing thesingle-conductor full-duplex two-way optical communication system 10,the sender-side fiber-coupled optical power S1_, which is the power oflight coupled into the optical fiber 11 from the first module 12 a, isadjusted according to Formula 18, in accordance with the far-endreflectivity FR_ on the side of the first module 12 a.

Alternatively, in a method of the present embodiment for manufacturingthe single-conductor full-duplex two-way optical communication system10, the sent light transmittance PT1_, which is the transmittance of theoptical fiber 11 with respect to the first signal light s1 emitted fromthe first module 12 a, is adjusted according to Formula 19, inaccordance with the far-end reflectivity FR_ on the side of the firstmodule 12 a.

In these manufacturing methods, first, the far-end reflectivity, FR_ onthe side of the first module 12 a is found by using actual members ofthe single-conductor full-duplex two-way optical communication system10. Then, the sender-side fiber-coupled optical power S1_ or the sentlight transmittance PT1_ is adjusted in accordance with the far-endreflectivity FR_ found.

The following more specifically describes advantages of adjusting thesender-side fiber-coupled optical power S1_ or the sent lighttransmittance PT1_ in accordance with the far-end reflectivity FR_.

It is confirmed that, in many kinds of optical systems, there is atrade-off between (i) a receiving efficiency at a receiver with respectto light emitted from an optical fiber and (ii) a far-end reflectivity.This is shown in FIG. 7, which illustrates an example of a relationshipbetween the receiving efficiency and the far-end reflectivity. In FIG.7, the relationship between the receiving efficiency and the far-endreflectivity of the receiver is ascertained under a condition that a 1mm-diameter POF is used, and an emission NA is 0.35, the emission NAbeing expressed as an angle that gives a half value of a peak intensityof a light beam at the end of the optical fiber.

The reason for this trade-off is described with reference to FIGS. 8( a)and 8(b). FIGS. 8( a) and 8(b) explains with illustration that there isthe trade-off between (i) the receiving efficiency at a receiver 22 withrespect to the light emitted from the optical fiber 21 and (ii) thefar-end reflectivity.

In order to lower the far-end reflectivity by preventing a reflectedlight s22 of an emitted light s21 from geometrically returning into theoptical fiber 21, an effective measure is to slant a light-receivingsurface 22 a of the receiver 22. However, if the light-receiving surface22 a is slanted, a substantive light-receiving area of the receiver 22for receiving the emitted light s21 is decreased. That is, a projectionarea of the emitted light 22 a on the receiver 22 is increased.Therefore, it can be said that there is the trade-off between (i) thereceiving efficiency at the receiver with respect to the light emittedfrom the optical fiber and (ii) the far-end reflectivity.

Discussed next is how the receiving efficiency and the far-endreflectivity influence the signal-noise ratio SN (the eye opening ratioIO). As seen from Formula 18, whereas the far-end reflectivity FR_influences only on one term, the receiving efficiency (the minimumreceiving efficiency Rmin in formula 18) influences two terms.Therefore, it is expected that the receiving efficiency will have agreater influence on the signal-noise ratio SN.

This is confirmed in FIGS. 9( a) and 9(b). FIGS. 9( a) and 9(b) aregraphs showing influences of the receiving efficiency and the far-endreflectivity on the eye opening ratio IO in a test product that performstransmission at 500 Mbps. FIG. 9( a) is about a case in which thefar-end reflectivity is 1.4% (a reflectivity at the receiver: 0.7%; areflectivity on a receiver-side end surface of the optical fiber: 0.7%).FIG. 9( b) is about a case in which the far-end reflectivity is 2.1%(the reflectivity at the receiver: 1.4%; the reflectivity on thereceiver-side end surface of the optical fiber: 0.7%). When FIG. 9( a)and FIG. 9( b) are compared, it is found that the far-end reflectivityin FIG. 9( b) is 1.5 times higher than the far-end reflectivity in FIG.9( a). However, in order to attain the same eye opening 0.8 ns in FIGS.9( a) and (b), it is sufficient to improve the receiving efficiency onlyby 0.2 dB (0.1%, from −5.9 dB to −5.7 dB.

Therefore, it can be said that, even if the far-end reflectivity on thereceiver side is sacrificed, it is preferable to increase the receivingefficiency of the optical system in order to obtain a receiver of higherperformance (better signal-noise ratio, higher eye opening IO, and lowerBER).

At this stage, the single-conductor full-duplex two-way opticalcommunication system 10 is considered. It should be noted that, whereasthe signal-noise ratio SN at the second module 12 b can be improved byimproving the receiving efficiency at the second module 12 b, whether toincrease or decrease the far-end reflectivity on the side of the secondmodule 12 b (the far-end reflectivity on the side of the second module12 b seen from the first module 12 a) has no influence on thesignal-noise ratio SN at the second module 12 b. The far-endreflectivity on the side of the second module 12 b does not influencethe optical signal received by the second module 12 b, but influencesthe optical signal received by the first module 12 a. That is, thesignal-noise ratio SN at the second module 12 b is influenced by thereceiving efficiency at the second module 12 b and the far-endreflectivity on the side of the first module 12 a.

Under such a relationship, if the far-end reflectivity on the side ofthe first module 12 a and the far-end reflectivity on the side of thesecond module 12 b are adjusted to predetermined values in advance, thereceiving efficiency, which significantly influences the signal-noiseratio SN, is sacrificed because the receiving efficiency is constrainedby the far-end reflectivity. This can be an obstacle in improving thesignal-noise ratio SN. The same is almost equally true with arelationship between the far-end reflectivity and the sent lighttransmittance.

In view of the circumstances, in a manufacturing method of the presentinvention, priority is given to the receiving efficiency at the firstmodule 12 a with respect to the light emitted from the optical fiber 11,and a position of the first module 12 a with respect to the opticalfiber 11 is determined in accordance with the receiving efficiency.Specifically, for example, the position of the first module 12 a(especially of the receiving section 14 a) with respect to the opticalfiber 11 is determined so as to maximize the receiving efficiency. As aresult, the position of the first module 12 a is the position as shownin FIG. 8( a), for example. Then, the far-end reflectivity FR_ in theposition is ascertained, and the sender-side fiber-coupled optical powerS1_ or the sent light transmittance PT1_ is set in accordance with thefar-end reflectivity FR_ so as to satisfy Formula 18 or Formula 19.

That is, although the far-end reflectivity on the sender side isincreased if priority is given to the receiving efficiency on the senderside, a loss of quality of the signal light received on the receiverside can be compensated by (i) adjusting the sender-side fiber-coupledoptical power on the sender side, or (ii) by performing an adjustment ofthe optical-fiber transmittance of the signal light sent from the senderside, the adjustment being performed by adjusting a property of thesignal light, for example. As a result, it is possible to manufacture asingle-conductor full-duplex two-way optical communication system 10that satisfies the requirement for the signal-noise ration SN (eyeopening IO and BER).

Moreover, the signal-noise ratio SN can be improved with a simplearrangement, such as the arrangement shown in FIG. 8( a), which issimpler than the arrangement shown in FIG. 8( b). Therefore, it ispossible to manufacture a single-conductor full-duplex two-way opticalcommunication system 10 that is advantageous in terms of a manufacturingcost.

Note that, although the foregoing explanation supposes that the firstmodule 12 a is the sender side and the second module 12 b is thereceiver side, the same setting may be adopted while supposing that thesecond module 12 b is the sender side and the first module 12 a is thereceiver side. In this case also it is possible to manufacture asingle-conductor full-duplex two-way optical communication system 10that as a whole satisfies the requirement for the signal-noise rationSN.

Second Embodiment

In the first embodiment, in accordance with the far-end reflectivity FR_on the side of the first module 12 a, (i) the sender-side fiber-coupledoptical power S1_, which is the power of light coupled into the opticalfiber 11 from the first module 12 a, is adjusted according to Formula18, or (ii) the sender-side transmittance PT1_, which is thetransmittance of the optical fiber 11 with respect to the first signallight s1 emitted from the first module 12 a, is adjusted according toFormula 19.

In this case, it is necessary to ascertain the far-end reflectivity ofeach module, and to individually adjust the sender-side fiber-coupledoptical power or the optical-fiber transmittance with respect to thesignal light.

However, ranges of variation of the sender-side fiber-coupled opticalpower and of the optical-fiber transmittance of the signal light aredetermined by what kinds of optical system and of light source are usedin the sending section 13 a of the first module 12 a (the sender side).There is no problem if the variation falls within the ranges acceptablein the single-conductor full-duplex two-way optical communication system10, that is, if the acceptable minimum fiber-coupled optical power Sminand the acceptable minimum transmittance PTmin are satisfied.

For example, in the foregoing example, the minimum fiber transmittanceof the 10 m-optical fiber is −5.1 dB if an LED is used as the lightsource of the sending section 13 a of the first module 12 a, whereas theminimum fiber transmittance is −3.1 dB if an LD is used as the lightsource of the sending section 13 a of the first module 12 a. The minimumfiber-coupled optical power is −10.9 dB if an LED is used, whereas theminimum fiber-coupled optical power is between −7 dB and −10 dB if an LDis used. As described above, the minimum fiber transmittance and theminimum fiber-coupled optical power are determined by a kind andspecification of the light source, and by the optical system.

Therefore, in a method of the present invention for manufacturing thesingle-conductor full-duplex two-way optical communication system 10, agroup of modules used as the first module 12 a are determined inaccordance with (i) the far-end reflectivity and (ii) the sender-sideminimum fiber-coupled optical power S1min or the sent light minimumtransmittance PT1min in a group of modules of the same kind.

Here, the sender-side minimum fiber-coupled optical power S1min is aminimum value among various values of the power of light coupled intothe optical fiber 11 from the group of modules of the same kind. Thesent light minimum transmittance PT1min is a minimum value among variousvalues of the transmittance of the optical fiber 11 with respect to thesignal light emitted from the group of modules of the same kind. Themodules of the same kind are modules uniformly designed in terms ofparticular properties (here, (i) an optical power property relating tothe power of light coupled, (ii) a wavelength relating to thetransmittance, and the like).

If “Smin” in Formula 17 is considered as a variable (the sender-sideminimum fiber-coupled optical power S1min_), Formula 17 can be expressedas Formula 20. If “PTmin” in Formula 17 is considered as a variable (thesent light minimum transmittance PT1min_), Formula 17 can be expressedas Formula 21.

$\begin{matrix}\begin{matrix}{{(a)\mspace{14mu}{If}\mspace{14mu}{IO}} \geq 0.3} \\{\mspace{34mu}{\frac{\left( {1 - {IO}} \right)}{0.7} > \frac{{S\mspace{11mu}\max*\left( {\frac{NR}{R\mspace{11mu}\min} + {{FR\_}*{PT}\mspace{11mu}\max^{2}}} \right)} + {X*\frac{Namp}{\;{R\mspace{11mu}\min}}}}{{S1}\mspace{11mu}{min\_}*{PT}\mspace{11mu}\min}}} \\{{(b)\mspace{14mu}{If}\mspace{14mu}{IO}} < 0.3} \\{\mspace{34mu}{1 > \frac{{S\mspace{11mu}\max*\left( {\frac{NR}{R\mspace{11mu}\min} + {{FR\_}*{PT}\mspace{11mu}\max^{2}}} \right)} + {X*\frac{Namp}{\;{R\mspace{11mu}\min}}}}{S\mspace{11mu}{min\_}*{PT}\mspace{11mu}\min}}}\end{matrix} & (20) \\\begin{matrix}{{(a)\mspace{14mu}{If}\mspace{14mu}{IO}} \geq 0.3} \\{\mspace{34mu}{\frac{\left( {1 - {IO}} \right)}{0.7} > \frac{{S\mspace{11mu}\max*\left( {\frac{NR}{R\mspace{11mu}\min} + {{FR\_}*{PT}\mspace{11mu}\max^{2}}} \right)} + {X*\frac{Namp}{\;{R\mspace{11mu}\min}}}}{{S\mspace{11mu}\min*{PT1}\mspace{11mu}{min\_}}\;}}} \\{{(b)\mspace{14mu}{If}\mspace{14mu}{IO}} < 0.3} \\{\mspace{34mu}{1 > \frac{{S\mspace{11mu}\max*\left( {\frac{NR}{R\mspace{11mu}\min} + {{FR\_}*{PT}\mspace{11mu}\max^{2}}} \right)} + {X*\frac{Namp}{\;{R\mspace{11mu}\min}}}}{S\mspace{11mu}\min*{PT1}\mspace{11mu}{min\_}}}}\end{matrix} & (21)\end{matrix}$

With respect to plural groups of modules, the modules having apossibility of being used as the first module 12 a and the modules beingof different kinds from group to group, the sender-side minimumfiber-coupled optical power S1min or the sent light minimumtransmittance PT1min is determined in advance with respect to eachgroup. Then, in accordance with the far-end reflectivity FR_ on the sideof the first module 12 a, a group in which the S1min_ satisfies formula20 or the PT1min_ satisfies Formula 21 is selected. By using, as thefirst module 12 a, the modules included in the selected group, it ispossible to manufacture a single-conductor full-duplex two-way opticalcommunication system 10 that satisfies the requirement for thesignal-noise ratio SN.

Note that, as in the first embodiment, the position of the first module12 a is determined with priority to the receiving efficiency of theoptical fiber 11 with respect to the light emitted into the first module12 a.

In Formula 20 or Formula 21, the range of the acceptable far-endreflectivity is narrower than in Formula 18 or Formula 19. However, itis possible to save time and labor to adjust the properties of themodules individually.

In the first embodiment or the second embodiment, (i) either one of thesender-side fiber-coupled optical power S1_ and the sender-side minimumfiber-coupled optical power S1min_ and (ii) either one of the sent lighttransmittance PT1_ and the sent light minimum transmittance PT1min_ areconsidered to be variables, respectively. Alternatively, however, thefollowing may be considered to be variables, respectively: (thesender-side fiber-coupled optical power S1_)×(the sent lighttransmittance PT1_), (the sender-side minimum fiber-coupled opticalpower S1min_)×(the sent light minimum transmittance PT1min_), (thesender-side fiber-coupled optical power S1_)×(the sent light minimumtransmittance PT1min_), and (the sender-side minimum fiber-coupledoptical power S1min_)×(the sent light transmittance PT1_).

As described above, in a method of the present invention formanufacturing an optical communication system including (i) an opticalfiber and (ii) first and second modules respectively provided at bothends of the optical fiber, the first and second modules being capable ofsimultaneously sending and receiving optical signals via the opticalfiber, S1_ or PT1_ is set in accordance with a value of FR_ so as tosatisfy Formula 18 or Formula 19.

With this method, by giving priority to determining a condition thatsignificantly influences an improvement of the eye opening ratio, it ispossible to manufacture an optical communication system at a low costand with more freedom in manufacturing.

In a method of the present invention for manufacturing an opticalcommunication system including (i) an optical fiber and (ii) first andsecond modules respectively provided at both ends of the optical fiber,the first and second modules being capable of simultaneously sending andreceiving optical signals via the optical fiber, modules included in agroup that satisfies Formula 20 or Formula 21 are used as the firstmodule in accordance with a value of FR_.

With this method, it is possible to save time and labor to set theproperties of the modules individually.

The invention being thus described, it will be obvious that the same waymay be varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

INDUSTRIAL APPLICABILITY

The present invention can be used for manufacturing an opticalcommunication system including (i) an optical fiber and (ii) modulesrespectively provided at both ends of the optical fiber, the modulesbeing capable of simultaneously sending and receiving optical signalsvia the optical fiber.

1. A method of manufacturing an optical communication system including(i) an optical fiber and (ii) first and second modules respectivelyprovided at both ends of the optical fiber, the first and second modulessimultaneously sending and receiving optical signals via the opticalfiber, wherein: a position of the first module with respect to theoptical fiber is determined in accordance with a receiving efficiency atthe first module with respect to light emitted from the optical fiber;and S1_ is set in accordance with a value of FR_ in the position so asto satisfy $\begin{matrix}\begin{matrix}{{(a)\mspace{14mu}{If}\mspace{14mu}{IO}} \geq 0.3} \\{\mspace{34mu}{\frac{\left( {1 - {IO}} \right)}{0.7} > \frac{{S\mspace{11mu}\max*\left( {\frac{NR}{R\mspace{11mu}\min} + {{FR\_}*{PT}\mspace{11mu}\max^{2}}} \right)} + {X*\frac{Namp}{\;{R\mspace{11mu}\min}}}}{{{S1\_}*{PT}\mspace{11mu}\min}\;}}} \\{{(b)\mspace{14mu}{If}\mspace{14mu}{IO}} < 0.3} \\{\mspace{34mu}{1 > \frac{{S\mspace{11mu}\max*\left( {\frac{NR}{R\mspace{11mu}\min} + {{FR\_}*{PT}\mspace{11mu}\max^{2}}} \right)} + {X*\frac{Namp}{\;{R\mspace{11mu}\min}}}}{S\; 1\_*{PT}\mspace{11mu}\min}}}\end{matrix} & \;\end{matrix}$ where FR_ is a far-end reflectivity, which is areflectivity of light that is emitted from the second module and isreflected (i) by the first module and (ii) by the first-module-side endof the optical fiber; S1_ is the power of light coupled into the opticalfiber from the first module; Smax is a maximum value acceptable in theoptical communication system of the power of light coupled into theoptical fiber; PTmin is a minimum value acceptable in the opticalcommunication system of a transmittance of the optical fiber withrespect to the optical signals; PTmax is a maximum value acceptable inthe optical communication system of the transmittance of the opticalfiber with respect to the optical signals; NR is a ratio, with respectto Smax, of a stray light component received by the second module, thestray light component being generated on the second-module-side end ofthe optical fiber and in the second module when light coupled into theoptical fiber with power of Smax is emitted from the second module; Rminis a minimum receiving efficiency at the second module with respect tolight emitted from the optical fiber; Namp is a light amountcorresponding to a noise in an amplifier for converting, into anelectric signal, an optical signal received by the second module; IO isan eye opening ratio required for the electric signal obtained byconversion through the amplifier; and X is a ratio, with respect toNamp, of an optical signal received by the second module when a biterror rate is in an upper limit value acceptable in the opticalcommunication system, and when there is no reflected light returning tothe second module after being emitted from the second module.
 2. Amethod of manufacturing an optical communication system including (i) anoptical fiber and (ii) first and second modules respectively provided atboth ends of the optical fiber, the first and second modulessimultaneously sending and receiving optical signals via the opticalfiber, wherein: a position of the first module with respect to theoptical fiber is determined in accordance with a receiving efficiency atthe first module with respect to light emitted from the optical fiber;the method comprising selecting, in accordance with a value of FR insaid position, from plural groups of modules, the modules beingdifferent from group to group, a group in which S1mn_ satisfies,$\begin{matrix}\begin{matrix}{{(a)\mspace{14mu}{If}\mspace{14mu}{IO}} \geq 0.3} \\{\mspace{34mu}{\frac{\left( {1 - {IO}} \right)}{0.7} > \frac{{S\mspace{11mu}\max*\left( {\frac{NR}{R\mspace{11mu}\min} + {{FR\_}*{PT}\mspace{11mu}\max^{2}}} \right)} + {X*\frac{Namp}{\;{R\mspace{11mu}\min}}}}{{{S1min\_}*{PT}\mspace{11mu}\min}\;}}} \\{{(b)\mspace{14mu}{If}\mspace{14mu}{IO}} < 0.3} \\{\mspace{34mu}{1 > \frac{{S\mspace{11mu}\max*\left( {\frac{NR}{R\mspace{11mu}\min} + {{FR\_}*{PT}\mspace{11mu}\max^{2}}} \right)} + {X*\frac{Namp}{\;{R\mspace{11mu}\min}}}}{S\; 1{min\_}*{PT}\mspace{11mu}\min}}}\end{matrix} & \;\end{matrix}$ and using one of the plural modules included in theselected group as the first module, where FR_ is a far-end reflectivity,which is a reflectivity of light that is emitted from the second moduleand is reflected (i) by the first module and (ii) by thefirst-module-side end of the optical fiber; S1min_ is a minimum value ofpower of light coupled into the optical fiber from the selected group ofmodules used as the first module; Smax is a maximum value in the opticalcommunication system of the power of light coupled into the opticalfiber; PTmin is a minimum value in the optical communication system of atransmittance of the optical fiber with respect to the optical signals;PTmax is a maximum value in the optical communication system of thetransmittance of the optical fiber with respect to the optical signals;NR is a ratio, with respect to Smax, of a stray light component receivedby the second module, the stray light component being generated on thesecond-module-side end of the optical fiber and in the second modulewhen light coupled into the optical fiber with power of Smax is emittedfrom the second module; Rmin is a minimum receiving efficiency at thesecond module with respect to light emitted from the optical fiber; Nampis a light amount corresponding to a noise in an amplifier forconverting, into an electric signal, an optical signal received by thesecond module; IO is an eye opening ratio required for the electricsignal obtained by conversion through the amplifier; and X is a ratio,with respect to Namp, of an optical signal received by the second modulewhen a bit error rate is in an upper limit value acceptable in theoptical communication system, and when there is no reflected lightreturning to the second module after being emitted from the secondmodule.
 3. A method of manufacturing an optical communication systemincluding (i) an optical fiber and (ii) first and second modulesrespectively provided at both ends of the optical fiber, the first andsecond modules simultaneously sending and receiving optical signals viathe optical fiber, wherein: a position of the first module with respectto the optical fiber is determined in accordance with a receivingefficiency at the first module with respect to light emitted from theoptical fiber; and PT1_ is set in accordance with a value of FR_ in theposition so as to satisfy $\begin{matrix}\begin{matrix}{{(a)\mspace{14mu}{If}\mspace{14mu}{IO}} \geq 0.3} \\{\mspace{34mu}{\frac{\left( {1 - {IO}} \right)}{0.7} > \frac{{S\mspace{11mu}\max*\left( {\frac{NR}{R\mspace{11mu}\min} + {{FR\_}*{PT}\mspace{11mu}\max^{2}}} \right)} + {X*\frac{Namp}{\;{R\mspace{11mu}\min}}}}{{S\mspace{11mu}\min*{PT1\_}}\;}}} \\{{(b)\mspace{14mu}{If}\mspace{14mu}{IO}} < 0.3} \\{\mspace{34mu}{1 > \frac{{S\mspace{11mu}\max*\left( {\frac{NR}{R\mspace{11mu}\min} + {{FR\_}*{PT}\mspace{11mu}\max^{2}}} \right)} + {X*\frac{Namp}{\;{R\mspace{11mu}\min}}}}{S\mspace{11mu}\min*{PT1\_}}}}\end{matrix} & \;\end{matrix}$ where FR_ is a far-end reflectivity, which is areflectivity of light that is emitted from the second module and isreflected (i) by the first module and (ii) by the first-module-side endof the optical fiber; PT1_ is a transmitivity of the optical fiber withrespect to light emitted from the first module; Smin is a minimum valuein the optical communication system of power of light coupled into theoptical fiber; Smax is a maximum value in the optical communicationsystem of the power of light coupled into the optical fiber; PTmax is amaximum value in the optical communication system of a transmittance ofthe optical fiber with respect to the optical signals; NR is a ratio,with respect to Smax, of a stray light component received by the secondmodule, the stray light component being generated on thesecond-module-side end of the optical fiber and in the second modulewhen light coupled into the optical fiber with power of Smax is emittedfrom the second module; Rmin is a minimum receiving efficiency at thesecond module with respect to light emitted from the optical fiber; Nampis a light amount corresponding to a noise in an amplifier forconverting, into an electric signal, an optical signal received by thesecond module; IO is an eye opening ratio required for the electricsignal obtained by conversion through the amplifier; and X is a ratio,with respect to Namp, of an optical signal received by the second modulewhen a bit error rate is in an upper limit value acceptable in theoptical communication system, and when there is no reflected lightreturning to the second module after being emitted from the secondmodule.
 4. A method of manufacturing an optical communication systemincluding (i) an optical fiber and (ii) first and second modulesrespectively provided at both ends of the optical fiber, the first andsecond modules simultaneously sending and receiving optical signals viathe optical fiber, wherein: a position of the first module with respectto the optical fiber is determined in accordance with a receivingefficiency at the first module with respect to light emitted from theoptical fiber; the method comprising selecting, in accordance with avalue of FR_ in said position, from plural groups of modules, themodules being different from group to group, a group in which PT1min_satisfies, $\begin{matrix}\begin{matrix}{{(a)\mspace{14mu}{If}\mspace{14mu}{IO}} \geq 0.3} \\{\mspace{34mu}{\frac{\left( {1 - {IO}} \right)}{0.7} > \frac{{S\mspace{11mu}\max*\left( {\frac{NR}{R\mspace{11mu}\min} + {{FR\_}*{PT}\mspace{11mu}\max^{2}}} \right)} + {X*\frac{Namp}{\;{R\mspace{11mu}\min}}}}{{S\mspace{11mu}\min*{PT1}\mspace{11mu}{min\_}}\;}}} \\{{(b)\mspace{14mu}{If}\mspace{14mu}{IO}} < 0.3} \\{\mspace{34mu}{1 > \frac{{S\mspace{11mu}\max*\left( {\frac{NR}{R\mspace{11mu}\min} + {{FR\_}*{PT}\mspace{11mu}\max^{2}}} \right)} + {X*\frac{Namp}{\;{R\mspace{11mu}\min}}}}{S\mspace{11mu}\min*{PT1}\mspace{11mu}{min\_}}}}\end{matrix} & \;\end{matrix}$ and using one of the plural modules included in theselected group as the first module, where FR_ is a far-end reflectivity,which is a reflectivity of light that is emitted from the second moduleand is reflected (i) by the first module and (ii) by thefirst-module-side end of the optical fiber; PT1min_ is a minimum valueof a transmittance of the optical fiber with respect to light emittedfrom the selected group of modules used as the first module; Smin is aminimum value in the optical communication system of power of lightcoupled into the optical fiber; Smax is a maximum value in the opticalcommunication system of the power of light coupled into the opticalfiber; PTmax is a maximum value in the optical communication system of atransmittance of the optical fiber with respect to the optical signals;NR is a ratio, with respect to Smax, of a stray light component receivedby the second module, the stray light component being generated on thesecond-module-side end of the optical fiber and in the second modulewhen light coupled into the optical fiber with power of Smax is emittedfrom the second module; Rmin is a minimum receiving efficiency at thesecond module with respect to light emitted from the optical fiber; Nampis a light amount corresponding to a noise in an amplifier forconverting, into an electric signal, an optical signal received by thesecond module; IO is an eye opening ratio required for the electricsignal obtained by conversion through the amplifier; and X is a ratio,with respect to Namp, of an optical signal received by the second modulewhen a bit error rate is in an upper limit value acceptable in theoptical communication system, and when there is no reflected lightreturning to the second module after being emitted from the secondmodule.
 5. The method of manufacturing an optical communication systemas set forth in any one of claims 1 to 4, wherein: the optical fiber isa plastic optical fiber.